The couple, both 42, have been charged with murder and a lesser allegation of child endangerment for their treatment of their seven-year-old son, who also went without food.

The pair, who were married a year ago, face 60 years behind bars if found guilty, and remain in custody in Jersey County jail after a judge set bail at $500,000 each.

Police said they were called to a hospital in the southern Illinois town of Jerseyville on Friday (27 October) after the six-year-old boy, who was brought there by his father, had died.

Doctors told detectives the youngster tipped the scales at just 17 pounds, about a third of what a typical child of his age should weigh.

The boy's preliminary autopsy found him to be "extremely malnourished". The authorities are still waiting on the results of a toxicology test.

The complaint lodged in court against the couple is that from December 2015 they "knowingly and intentionally caused [the boy] to become extremely malnourished, leaving the said 6-year-old child... to starve to death," reported the St. Louis Post-Dispatch.

The youngster was one of six children who lived with the Roberts in Jerseyville, about 40 miles north of St. Louis. Police told the court they found the children living in "sickening" conditions.

"I had no idea that there was supposedly six children living in the house at all," neighbour Lucas Dickerman told broadcaster FOX2. "They never brought them outside to play. They never brought them over to say hi or anything. Very secretive people."

The state's Department of Children and Family Services has taken four of the children in protective custody. The Roberts are next expected to appear in court on 13 November.